I'm not sure how they are going to land when switching to the Merlin 2 engines. However I am quite sure they are will not give up on reusability, since that would be the same as signing their own death certificate, Others are considering different levels of reusability, and getting that part right is key, to staying competitive.

One crazy idea would be a cold gas mode, where the turbopump is running but the chamber is not lit. No idea if that is even conceivable and would yield an appropriate amount of thrust, but this is the Internet, so there you go .

Perhaps more importantly, it doesn't have to hover, all it has to do is decelerate to a standstill an inch above the pad. If your minimum force is high, that just means that you have to have a shorter, higher-G deceleration. So rather than gently floating to the pad, landing will be more like coming to a screeching halt at the last moment. That would be very high-G though. RGClark used an estimate of 13600 kg for the dry weight of the F9 first stage, and the Merlin 2 is supposed to have about 7600kN maximum thrust. Throttled to say 50% that's 3800 kN, giving an acceleration of a = F / m = (3800000 kg m / s^2) / 13600 kg = 280 m/s^2 = 29 G, with 1 G counteracted by gravity, so that's 28 G. That means coming down from just under supersonic speed to a standstill in a second. That cowboy on Grasshopper would get crushed, but would the stage survive?

On ascent, I think the F9 pulls at most 3G, which means that the first stage has to withstand at least 4 times (including gravity) the weight of the upper stage plus payload on ascent. Again going by RGClark's numbers, that's about 70000 kg * 4 * 9.8 m/s^2 = 2700 kN. IIRC, F9 has a structural safety margin of 1.7, which would give it a breaking limit of 1.7 * 2700 kN = 4600 kN, which is more than those 3800 kN.

Conclusion, unless I miscomputed something, it seems to be actually possible to slow down an F9 first stage to a standstill using a 50% throttled Merlin 2 without breaking it. The F9 1.1 first stage is larger and heavier, so that would improve the numbers a bit more. It's going to be one heck of a sight though...

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus

If you could put enough fuel in the rocket (or maybe have in space refueling ), maybe the rocket could decelerate in space, while at the same time holding it's altitude, more or less. That way, you wouldn't need a heat shield, because you wouldn't experience they incredible heat of slowing down in the atmosphere, and the dangers that come with it.

_________________“Once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return.” -Anonymous

If you could put enough fuel in the rocket (or maybe have in space refueling ), maybe the rocket could decelerate in space, while at the same time holding it's altitude, more or less. That way, you wouldn't need a heat shield, because you wouldn't experience they incredible heat of slowing down in the atmosphere, and the dangers that come with it.

I think it's cheaper just to have a decent heat shield. Although even a vertical drop would need a heat shield from orbital altitude.

If you could put enough fuel in the rocket (or maybe have in space refueling ), maybe the rocket could decelerate in space, while at the same time holding it's altitude, more or less. That way, you wouldn't need a heat shield, because you wouldn't experience they incredible heat of slowing down in the atmosphere, and the dangers that come with it.

I think it's cheaper just to have a decent heat shield. Although even a vertical drop would need a heat shield from orbital altitude.

Well, assuming you had enough fuel, you could use the rocket's engines to keep the speed down to manageable levels.You are probably right about the heat shield being cheaper. But there are a few upsides to reentry this way:First, assuming it works, reentry would be a lot safer. Also, you would save mass by not having the heat shield. And it would make the whole operation simpler, considering that SpaceX is already working on a reusable rocket (have you seen the grasshopper videos?). And also also, you wouldn't be as limited as to a landing spot.

_________________“Once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return.” -Anonymous

If you could put enough fuel in the rocket (or maybe have in space refueling ), maybe the rocket could decelerate in space, while at the same time holding it's altitude, more or less. That way, you wouldn't need a heat shield, because you wouldn't experience they incredible heat of slowing down in the atmosphere, and the dangers that come with it.

I think it's cheaper just to have a decent heat shield. Although even a vertical drop would need a heat shield from orbital altitude.

The easy way for Earth landings would be to have a guided Rogallo wing with the rocket as the counter weight and come done a lot slower so no need for a decent heat shield and just have enough fuel for an upright landing at the end. I think modern materials technologies and robot control could do it fairly easily these days i am not sure why they dropped the idea for the Gemini missions but i think they did fly at least one supersonic in tests.

If you could put enough fuel in the rocket (or maybe have in space refueling ), maybe the rocket could decelerate in space, while at the same time holding it's altitude, more or less. That way, you wouldn't need a heat shield, because you wouldn't experience they incredible heat of slowing down in the atmosphere, and the dangers that come with it.

I think it's cheaper just to have a decent heat shield. Although even a vertical drop would need a heat shield from orbital altitude.

Well, assuming you had enough fuel, you could use the rocket's engines to keep the speed down to manageable levels.You are probably right about the heat shield being cheaper. But there are a few upsides to reentry this way:First, assuming it works, reentry would be a lot safer. Also, you would save mass by not having the heat shield. And it would make the whole operation simpler, considering that SpaceX is already working on a reusable rocket (have you seen the grasshopper videos?). And also also, you wouldn't be as limited as to a landing spot.

Remember that aero braking is 'free' from an energy point of view. Second stage re usability costs weight, and the only way it is feasible is take every advantage. A heat shield weights a lot less than the fuel required to decelerate the stage enough to not need a heatshield (and remember, you need one anyway on a vertical drop - even SS1 had some minimal heat shield, and that only went to 100km). I'm not convinced it's any safer either - Heatshields have no moving parts to go wrong for a start.

I figure they need to build and fly a bunch of Merlin 2s in the beginning anyway, to test them and bring production costs down.

And then they'll build FX, FXH and FXX. When they have those working they can try to do powered landing with those vehicles to reduce costs. If all that works, F9H should become obsolete, replaced by FX. F9 might still be viable. By having both Merlin 1 and 2 to choose from they can select the engine that gives the best cost/benefit.

An FXX doing a powered landing. Now that would be something. And I think there was this British guy who knew exactly what that would look like.

"The ships hung in the sky in much the same way that bricks don't."

Can't believe he's been gone for almost 12 years now. Rest in peace Douglas.

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus

That Merlin 2 was a proposal to NASA to build a heavy lift engine if NASA paid for its development. SpaceX was not planning to build it on its own. So I don't think that is part of SpaceX's current plans. But an interesting question still holds about the vertical landing of the Falcon 9 first stage: even with the current Merlins and shutting down all but one of them and with throttling you still have a 3 to 1 or above thrust to weight ratio, for the empty Falcon 9 stage. This means you can not hover. So how do you do a safe landing? There was a discussion of this on NasaSpaceFlight.com. The conclusion was that you supply short bursts which would allow you to cancel out the downward velocity even though you could not hover.

Bob Clark

_________________Single-stage-to-orbit was already shown possible 50 years ago with the Titan II first stage. Contrary to popular belief, SSTO's in fact are actually easy. Just use the most efficient engines and stages at the same time, and the result will automatically be SSTO.Blog: http://exoscientist.blogspot.com

That makes sense, to vary the duty cycle, but it does require very reliable and fast restart capability, including restarting at a precisely defined moment for the last burn. Can a rocket engine the size of the Merlin do that?

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus

That makes sense, to vary the duty cycle, but it does require very reliable and fast restart capability, including restarting at a precisely defined moment for the last burn. Can a rocket engine the size of the Merlin do that?

Good points. I'll look that up on that NasaSpaceFlight thread. Frankly I would just like to have the capability to hover even if it required putting smaller additional engines on the first stage.

Bob Clark

_________________Single-stage-to-orbit was already shown possible 50 years ago with the Titan II first stage. Contrary to popular belief, SSTO's in fact are actually easy. Just use the most efficient engines and stages at the same time, and the result will automatically be SSTO.Blog: http://exoscientist.blogspot.com

Hmm, I don't know. You don't need hover ability to do an abort and go around. I think the key issue is control: with a big power to weight ratio the timing of everything becomes really critical. On the other hand, you're never going to have an abundance of fuel landing a rocket vertically. Being able to do one emergency abort away from the pad and into the ocean may be the best you can do without losing too much of your payload capacity.

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus